3D topology is the way the lines flow in a 3D model. A good example to look at is a 3D head; a 3D head must have "good topology" if you're planning on animating it at a later stage, and good topology means the edges must run smoothly and flow so that it won't cause errors later, bad topology can cause many errors in the mesh when animating.

emm.... I wanted to ask, what things to consider when we want to create a topology?
what is the difference between organic and inorganic modeling ??
and what is most important in making organic modeling ??

organic models are living creatures, these models will tend to be animated and so topology is extremely important. Bad topology will lead to a model that will deform badly when you begin painting weights and start the rigging process.

Inorganic would be hard surface modeling such as weapons, envoronments, cars etc. Topology is very important in these areas also and as with organics the best course of action is to model in quads to allow for your object to look good when subdivided or mesh smoothed. This is more the case for highend work though as hard surface models in video game will allow much more room for manouvre in this area.